There are several prior art three-mirror optical systems currently in use or contemplated for use, but none of which are successful in solving all the difficulties presented by such optical systems usable both as a collimator and a telescope, and particularly as a collimator that requires a real, accessible exit pupil. The principal shortcomings of prior three-mirror optical systems include inaccessability of the exit pupil and/or image plane, a large central obscuration, or exessive dimensions. It should be noted that three mirror optical systems as well as two mirror optical systems should correct as much as possible for the four types of primary aberrations including aspherical aberration, coma, astigmatism, and field curvature. While two mirror configurations are not, per se, a part of the background of this invention it should be noted in passing that all practical configurations can maximally correct only two types of aberrations, usually spherical aberration and coma whereas three mirror optical systems are capable of correcting three or, as in the present invention, all four of such aberrations.
One example of a known state-of-the-art three mirror optical system, which is primarily a telescope, and is relevant to the present invention, is disclosed in U.S. Pat. No. 4,240,707 to Wetherell which shows an on-axis (i.e. on the optical axis) virtual (i.e. non-accessible) entrance pupil. The radiation from an off-axis (i.e. off the optical axis) primary mirror is reflected back to an on-axis convex secondary mirror which in turn reflects the radiation to an off-axis tertiary mirror. The radiation is then reflected from the tertiary mirror to a focal plane which is also off-axis.
More specifically, it can be seen that in Wetherell, the entrance pupil is a virtual entrance pupil. Consequently, the Wetherell system cannot operate as telescope or collimator when an accessible entrance or exit pupil is required. The foregoing is shown clearly in FIG. 3 of Wetherell.
When an optical system is employed as a collimator it is frequently important that the collimated beam remain in a fixed location at the entrance pupil of the optical system to be tested. As discussed above, such a fixed beam location at the entrance pupil of the telescope to be tested is not possible with Wetherell since the entrance pupil is a virtual one. Furthermore, in Wetherell there are no intermediate images, thereby making the suppression of stray light more difficult.
Another example of a relevant three-mirror system is shown in U.S. Pat. No. 4,265,510 to Cook wherein a primary mirror, a secondary mirror, and a tertiary mirror form an anastigmatic relayed image-forming optical system in which the image field is off the optical axis and the entrance pupil coincides with the primary mirror and an intermediate image is formed between the secondary and tertiary mirrors.
A primary object of the present invention is to provide a three-mirror optical system which has a real, accessible entrance (or exit) pupil and a wide, flat image field thereby making the system particularly suitable to be employed as a collimator as well as a telescope.
A second object of the invention is a three-mirror optical system which contains an intermediate image between the primary and secondary mirrors to facilitate the suppression of stray radiation.
A third object of the invention is to provide a three-mirror optical system with a pupil image on or close to the secondary mirror and further with the primary and tertiary mirrors both being off-axis to provide for an unobstructed collimated beam (i.e. with no hole therein) to pass through the exit (or entrance) pupil.
A fourth object of the invention is the improvement of three-mirror optical systems generally.